1. Technical Field of the Invention
This invention relates generally to motorcycle suspension components, and more specifically to rear suspension spring and shock absorber coupling geometries.
2. Background Art
Motorcycles have been provided with a variety of rear suspension types. Some “hard tail” motorcycles have no rear suspension, with a rear axle being directly coupled to a unitary and substantially rigid frame. Other motorcycles have two-piece frame structures, with a primary frame holding the engine, and a swingarm holding the rear axle. The front end of the swingarm is coupled to a swingarm pivot which is part of the frame or, in some cases, the engine. The swingarm pivots about this connection such that the rear axle is free to move substantially vertically up and down, enabling the rear tire to track bumps and holes in the road surface.
One or two springs are installed between the swingarm and the frame, to support the weight of the motorcycle and rider and thereby prevent the swingarm from collapsing to its uppermost position of travel. Most commonly, compression coil springs are used.
In order to dampen any oscillatory modes, and in order to absorb shock loads, a corresponding number of shock absorbers (sometimes referred to as dampers or dampeners or simply shocks) are also coupled between the swingarm and the frame. Most commonly, these are of the conventional valved hydraulic piston type.
Traditionally, a spring and its companion shock are combined into a single “coil over shock” unit, in which the shock is coaxially disposed within the coil spring, and in which the shock and spring are compressed and extended as a single unit. Typically, the ends of the coil spring are held in position by flanges built into the ends of the shock assembly, such that the two ends of the shock are respectively coupled to the swingarm and to the frame, and the coil spring itself is not directly coupled to the swingarm or the frame. Thus, the coil spring shares the shock's mounting points and is indirectly coupled to the frame and to the swingarm.
The term “spring rate” is generally used to describe the stiffness or strength of a spring, and is measured as the amount of force necessary to compress or extend the spring a predetermined distance. For example, a spring with a 40 kg/cm coil spring will compress an additional 1 cm for every extra 40 kg weight stacked atop it. Some coil springs are “progressively wound”, with some coils more closely spaced than others. The closer coils are more perpendicular to the axis of the spring, and are therefore “softer”.
The term “spring leverage rate” will be used in this disclosure to describe the amount of travel of a spring (or, in other words, the change in the axial length of the spring) relative to the amount of travel of a lever arm to which it is attached (specifically, the swingarm lever arm from the swingarm pivot to the rear axle). If the spring is compressed 1 cm for every 4.5 cm of rear axle travel, the suspension has a 4.5:1 spring leverage rate.
Similarly, the term “shock leverage rate” will be used in this disclosure to describe the amount of travel of a shock absorber (or, in other words, the change in the axial length of the shock absorber) relative to the amount of travel of the swingarm lever arm at the rear axle. If the shock is shortened 1 cm for every 3 cm of rear axle travel, the suspension has a 3:1 shock leverage rate.
If a spring leverage rate or a shock leverage rate does not vary non-trivially over the range of motion of the rear axle, it will be said to have a “linear rate”. If the leverage rate varies non-trivially over the range of motion of the rear axle, it will be said to have a “progressive rate”. If the leverage rate increases as the rear axle rises (meaning that the farther the rear axle moves, the greater the relative change in the shock or spring length is), the progressive rate is said to be a “rising rate”. If the leverage rate decreases as the rear axle rises (meaning that the farther the rear axle moves, the less relative change there is in the shock or spring length), the progressive rate is said to be a “falling rate”.
In some motorcycles, typically those from the 1960s such as the Norton Commando, the lower end of the coil-over shock was connected directly to the swingarm, and the upper end of the coil-over shock was connected directly to the frame. This results in a roughly linear leverage rate being applied to both the spring and the shock. In other words, the spring force is approximately directly proportional to the position of the rear axle, and the dampening force is approximately directly proportional to the velocity of the rear axle regardless of the position of the rear axle.
In other motorcycles, typically newer models such as the Ducati 999 or the Suzuki GSX-R, the coil-over shock is not directly connected to the swingarm (or, in a few cases, to the frame), but is connected via a “linkage” which includes a set of pivotably interconnected lever arms whose geometry is designed to give the coil-over shock a progressive leverage rate. In virtually all cases, the progressive leverage rate is a rising rate accompanied by large quantities of marketing hype.
In contrast, one author has suggested that the Kawasaki Uni-Trak system which was introduced in 1980 was a falling-rate suspension.
Some motorcycles have used a pair of coil-over shocks—one on each side of the rear wheel—while other motorcycles have used a single coil-over shock, a trend which began with late 1970's dirt bikes. To achieve the same spring power as a pair of springs, a single spring will generally have to be twice as heavy as either of the springs in the pair; in other words, the single spring will weigh approximately as much as the pair of springs together. However, to achieve the same dampening power as a pair of shock absorbers, a single shock absorber does not need to weigh twice as much and, in fact, may be the same weight or only slightly heavier than either one of the pair of shocks.
Unfortunately, if the shock absorber were eliminated from the coil-over shock on only one side of the swingarm, the swingarm would be subjected to significant torsional forces during operation of the motorcycle, as the dampening force would be applied asymmetrically to only the other side of the swingarm.
More significantly, having the springs and the shock absorbers operating in lock-step forces the designer to make undesirable tradeoffs and compromises in the linkage geometry.
What is needed, then, is an improved motorcycle suspension which decouples the springs from the shock absorbers, granting the designer greatly increased freedom in selecting linkage geometry and component placement.